The present invention relates to a heterojunction bipolar transistor for processing high-speed signals, and more particularly to a device structure well suited to raise the integration density and enhance the performance of the transistor and a method of producing the same.
The heterojunction bipolar transistor is characterized in that a base-emitter junction is formed using semiconductors with which the band gap of an emitter becomes wider than that of a base, thereby to reduce the injection of minority carriers from the base into the emitter. Thus, the efficiency of the injection of majority carriers from the emitter into the base is enhanced, and the impurity concentration of the base can be raised, so that a transistor of high current gain and low base resistance becomes possible.
The idea of the heterojunction bipolar transistor is old, and has been realized using a heterojunction of Ga.sub.x-1 Al.sub.x As-GaAs (Proceedings of the 12th conf on Solid State Devices, 1980, p. 1).
FIGS. 1(a) and 1(b) show the sectional structure and energy band structure of the active regions of the transistor, respectively. This transistor is constructed of an emitter 1 made of an n-type Ga.sub.0.7 Al.sub.0.3 As layer, a base 2 made of a p-type GaAs layer, and a collector made of an n-type GaAs layer 3 and an n.sup.+ -type GaAs layer 4. The band gap (5) of Ga.sub.0.7 Al.sub.0.3 As is 1.79 eV, which is approximately 0.37 eV wider than the band gap (6) of GaAs. Approximately 0.05 eV of the difference is allotted to a valence band and lowers the energy level of the emitter by this component (7), to suppress the injection (8) of holes.
At a hetero-interface on a conduction band side, a notch (9) of about 0.3 eV arises in the band gap and similarly reduces the injection (10) of electrons. As an expedient for eliminating the notch (9), it has been considered to gently change the mixed ratio in the vicinity of the hetero-interface (for example, Solid State Electron., Vol. 15, No. 12, p. 1339, ('72)).
In addition, a structure in which the base 2 is also formed of Ga.sub.1-x Al.sub.x As having a mixed ratio gradient has been considered as an expedient wherein electrons in the base 2 are caused to reach the collector region 3 by drift, not by diffusion, thereby to achieve an enhanced operating speed (Japanese Patent Application Laid-open No. 49-43583).
However, both of the known examples mentioned above concern the active regions of the transistor, and the way how a base electrode and a collector electrode are led out becomes important for enhancing the performance.
As an example whose portions for leading out the electrodes are most advanced, a transistor of a sectional structure shown in FIG. 2 has been developed ('84 ISSCC, Digest of Tech. Papers, p. 51 ('84)).
In this transistor, an n.sup.+ -type GaAs layer 12 is disposed on active layers in order to improve the property of the ohmic contact between an emitter electrode 13 and the emitter 1. Portions accessory to the active regions are principally formed of base lead-out portions 14 as well as base electrodes 15 and a collector electrode 16. The base lead-out portions 14 are made of p-type layers which are formed on both the sides of the active layers by ion implantation. The collector electrode 16 is formed directly on the exposed collector layer 4 after the stacked layers have been windowed down to the collector layer 4 by selective etching as indicated by numeral 17.
Numerals 18, 19 and 20 designate insulating layers which are all formed by ion implantation. The layers 18 isolate the collector and the base, the layers 19 isolate the emitter and the base, and the layers 20 isolate the individual elements (numeral 11 indicates a substrate).
In the heterojunction bipolar transistor of this structure, the collector electrode 16 is formed at a position about 0.3 .mu.m lower than the surface 21 of the device, to basically construct a mesa type structure, so that a high density of integration is difficult. Moreover, since the base lead-out portions are formed around the active regions, there are the problems that the area of the device enlarges and that the distance between the active regions and the collector electrode 16 lengthens to increase the series resistance.